The best thing to do is to put a circuit monitoring device on all of the 3 phases, and let it record for 30 days, which I believe is an NEC requirement if you are in the US. This will give you a baseline reading. Then you can determine if your panelboard can handle the new load.
I assume the 75A breaker is on the Incomer to the switchboard.
To determine if the existing 75A Incomer breaker is adequate for the propopsed increase in switchboard load :-
1.) Measure switchboard existing maximum demand current over 15 minute demand interval.
2.) Add to this max demand the max demand of the proposed new load. Continuous rating of Incomer must be greater than max demand.
3.) Check that the proposed max demand does not exceed the continous rating of main busbar in the switchboard.
4.) Calculate max prospective 3 phase short-circuit current at the switchboard. Interrupt rating of the breakers must be greater than the fault current.
Note: The breakers on the outgoing circuits can have interrupt rating less than prospective fault current only if the incomer and outgoing breakers are current limiting and chosen from the manufactuer's certified tables.
5.) Check grading is provided in the event of a fault downstream from the outgoing breakers.
